XPS and SERS study of silicon phthalocyanine monolayers: Umbrella vs octopus design strategies for formation of oriented SAMs

Two strategies are compared for the formation of self-assembled monolayers (SAMs) of silicon phthalocyanines on gold. Silicon plithalocyanines were sy nthesized with thiol anchoring groups in either eight peripheral side chain s (the "octopus") or with one short thiol in an axial position (the "umbrel la"). Both approaches gave phthalocyanines capable of forming SAMs on gold surfaces. The orientation and coverage of the plithalocyanines were compare d using ellipsometry, X-ray photoelectron spectroscopy (XPS), and surface-e nhanced Raman scattering (SERS). The octopus silicon plithalocyanines form poorly organized SAMs in which the phthalocyanine (Pc) rings are strongly t ilted with respect to the gold surface. On average, between 3 and 4 of the thiol "arms" fail to bind to the gold surface, even when limiting coverage is achieved after 7 days of soaking. The film thickness is 22 +/- 5 Angstro m. In contrast, the umbrella silicon phthalocyanine produces close-packed S AMs within 1 h in which the Pc rings lie parallel to the gold surface. The average thickness of the later SAMs is 11 +/- 3 Angstrom, and each phthaloc yanine ring occupies an average area of 284 Angstrom (2).